Pressure gradient CVI/CVD process

ABSTRACT

The invention relates to pressure gradient processes for forcing infiltration of a reactant gas into a porous structure. The process comprises the steps of partially densifying a porous structure with a pressure gradient CVI/CVD process in which a first potion of the porous structure is subjected to greater pressure than a second portion of the porous structure. The process is suited for the simultaneous CVI/CVD processing of large quantities of aircraft brake.

This application is a continuation of application Ser. No. 08/881,398,filed Jun. 24, 1997 (abandoned), which is a continuation of applicationSer. No. 08/340,510, Nov. 16, 1994, now abandoned, both of which arehereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

The invention relates to the field of high temperature composites madeby the chemical vapor infiltration and deposition of a binding matrixwithin a porous structure. More particularly, the invention relates topressure gradient processes for forcing infiltration of a reactant gasinto a porous structure, apparatus for carrying out those processes, andthe resulting products.

Chemical vapor infiltration and deposition (CVI/CVD) is a well knownprocess for depositing a binding matrix within a porous structure. Theterm “chemical vapor deposition” (CVD) generally implies deposition of asurface coating, but the term is also used to refer to infiltration anddeposition of a matrix within a porous structure. As used herein, theterm CVI/CVD is intended to refer to infiltration and deposition of amatrix within a porous structure. The technique is particularly suitablefor fabricating high a temperature structural composites by depositing acarbonaceous or ceramic matrix within a carbonaceous or ceramic porousstructure resulting in very useful structures such as carbon/carbonaircraft brake disks, and ceramic combustor or turbine components. Thegenerally known CVI/CVD processes may be classified into four generalcategories: isothermal, thermal gradient, pressure gradient, and pulsedflow. See W. V. Kotlensky, Deposition of Pyrolytic Carbon in PorousSolids, 8 Chemistry and Physics of Carbon, 173, 190-203 (1973); W. J.Lackey, Review, Status, and Future of the Chemical Vapor InfiltrationProcess for Fabrication of Fiber-Reinforced Ceramic Composites, Ceram.Eng. Sci. Proc. 10[7-8] 577, 577-81 (1989) (W. J. Lackey refers to thepressure gradient process as “isothermal forced flow”). In an isothermalCVI/CVD process, a reactant gas passes around a heated porous structureat absolute pressures as low as a few millitorr. The gas diffuses intothe porous structure driven by concentration gradients and cracks todeposit a binding matrix. This process is also known as “conventional”CVI/CVD. The porous structure is heated to a more or less uniformtemperature, hence the term “isothermal,” but this is actually amisnomer. Some variations in temperature within the porous structure areinevitable due to uneven heating (essentially unavoidable in mostfurnaces), cooling of some portions due to reactant gas flow, andheating or cooling of other portions due to heat of reaction effects. Inessence, “isothermal” means that there is no attempt to induce a thermalgradient that preferentially affects deposition of a binding matrix.This process is well suited for simultaneously densifying largequantities of porous articles and is particularly suited for makingcarbon/carbon brake disks. With appropriate processing conditions, amatrix with desirable physical properties can be deposited. However,conventional CVI/CVD may require weeks of continual processing in orderto achieve a useful density, and the surface tends to densify firstresulting in “seal-coating” that prevents further infiltration ofreactant gas into inner regions of the porous structure. Thus, thistechnique generally requires several surface machining operations thatinterrupt the densification process.

In a thermal gradient CVI/CVD process, a porous structure is heated in amanner that generates steep thermal gradients that induce deposition ina desired portion of the porous structure. The thermal gradients may beinduced by heating only one surface of a porous structure, for exampleby placing a porous structure surface against a susceptor wall, and maybe enhanced by cooling an opposing surface, for example by placing theopposing surface of the porous structure against a liquid cooled wall.Deposition of the binding matrix progresses from the hot surface to thecold surface. The fixturing for a thermal gradient process tends to becomplex, expensive, and difficult to implement for densifying relativelylarge quantities of porous structures.

In a pressure gradient CVI/CVD process, the reactant gas is forced toflow through the porous structure by inducing a pressure gradient fromone surface of the porous structure to an opposing surface of the porousstructure. Flow rate of the reactant gas is greatly increased relativeto the isothermal and thermal gradient processes which results inincreased deposition rate of the binding matrix. This process is alsoknown as “forced-flow” CVI/CVD. Prior fixturing for pressure gradientCVI/CVD tends to be complex, expensive, and difficult to implement fordensifying large quantities of porous structures. An example of aprocess that generates a longitudinal pressure gradient along thelengths of a bundle of unidirectional fibers is provided in S. Kamura,N. Takase, S. Kasuya, and E. Yasuda, Fracture Behaviour of C Fiber/CVD CComposite, Carbon '80 (German Ceramic Society) (1980). An example of aprocess that develops a pure radial pressure gradient for densifying anannular porous wall is provided in U.S. Pat. Nos. 4,212,906 and4,134,360. The annular porous wall disclosed by these patents may beformed from a multitude of stacked annular disks (for making brakedisks) or as a unitary tubular structure. For thick-walled structuralcomposites, a pure radial pressure gradient process generates a verylarge, undesirable density gradient from the inside cylindrical surfaceto the outside cylindrical surface of the annular porous wall. Also, thesurface subjected to the high pressure tends to densify very rapidlycausing that surface to seal and prevent infiltration of the reactantgas to low density regions. This behavior seriously limits the utilityof the pure radial pressure gradient process.

Finally, pulsed flow involves rapidly and cyclically filling andevacuating a chamber containing the heated porous structure with thereactant gas. The cyclical action forces the reactant gas to infiltratethe porous structure and also forces removal-of the cracked reactant gasby-products from the porous structure. The equipment to implement such aprocess is complex, expensive, and difficult to maintain. This processis very difficult to implement for densifying large numbers of porousstructures.

Many workers in the art have combined the thermal gradient and pressuregradient processes resulting in a “thermal gradient-forced flow”process. Combining the processes appears to overcome the shortcomings ofeach of the individual processes and results in very rapid densificationof porous structures. However, combining the processes also results intwice the complexity since fixturing and equipment must be provided toinduce both thermal and pressure gradients with some degree of control.A process for densifying small disks and tubes according to a thermalgradient-forced flow process is disclosed by U.S. Pat. No. 4,580,524;and by A. J. Caputo and W. J. Lackey, Fabrication of Fiber-ReinforcedCeramic Composites by Chemical Vapor Infiltration, Prepared by the OAKRIDGE NATIONAL LABORATORY for the U.S. DEPARTMENT OF ENERGY underContract No. DE-AD05-840R21400 (1984). According to this process, afibrous preform is disposed within a water cooled jacket. The top of thepreform is heated and a gas is forced to flow through the preform to theheated portion where it cracks and deposits a matrix. A process fordepositing a matrix within a tubular porous structure is disclosed byU.S. Pat. No. 4,895,108. According to this process, the outercylindrical surface of the tubular porous structure is heated and theinner cylindrical surface is cooled by a water jacket. The reactant gasis introduced to the inner cylindrical surface. Similar forcedflow-thermal gradient processes for forming various articles aredisclosed by T. Hunh, C. V. Burkland, and B. Bustamante, Densificationof a Thick Disk Preform with Silicon Carbide Matrix by a CVI Process,Ceram. Eng. Sci. Proc 12[9-10] pp. 2005-2014 (1991); T. M. Besmann, R.A. Lowden, D. P. Stinton, and T. L. Starr, A Method for Rapid ChemicalVapor Infiltration of Ceramic Composites, Journal De Physique, ColloqueC5, supplement au n'5, Tome 50 (1989); T. D. Gulden, J. L. Kaae, and K.P. Norton, Forced-Flow Thermal-Gradient Chemical Vapor Infiltration(CVI) of Ceramic Matrix Composites, Proc.-Electrochemical Society(1990), 90-12 (Proc. Int. Conf. Chem. Vap. Deposition, 11th, 1990)546-52. Each of these disclosures describes processes for densifyingonly one porous article at a time, which is impractical forsimultaneously processing large numbers of composite articles such ascarbon/carbon brake disks.

In spite of these advances, a CVI/CVD process and an apparatus forimplementing that process are desired that rapidly and uniformlydensifies porous structures while minimizing cost and complexity. Such aprocess would preferably be capable of simultaneously densifying largenumbers (as many as hundreds) of individual porous structures. Inparticular, a process is desired for rapidly and economically densifyinglarge numbers of annular fibrous preform structures for aircraft brakedisks having desirable physical properties.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a CVI/CVD process is provided,comprising the steps of:

partially densifying a porous structure within a CVI/CVD furnace bydepositing a first matrix within the porous structure with a pressuregradient CVI/CVD process in which a first portion of the porousstructure is subjected to a greater pressure than a second portion ofthe porous structure and the first portion has a greater bulk densitygain than the second portion; and,

subsequently densifying the porous structure by depositing a secondmatrix within the porous structure with at least one additionaldensification process in which the second portion has a greater bulkdensity gain than the first portion.

According to another aspect of the invention, a CVI/CVD process isprovided, comprising the steps of:

partially densifying a multitude of annular fibrous carbon structureswithin a CVI/CVD furnace by depositing a first carbon matrix within theannular fibrous carbon structure with a pressure gradient CVI/CVDprocess in which a first portion of each annular fibrous carbonstructure is subjected to a greater pressure than a second portion ofeach annular fibrous carbon structure and the first portion has agreater bulk density gain than the second portion; and,

subsequently densifying the multitude of annular fibrous carbonstructures by depositing a second carbonaceous matrix within eachannular fibrous carbon structure with at least one additionaldensification process in which the second portion has a greater bulkdensity gain than the first portion.

According to yet another aspect of the invention, a friction disk isprovided, comprising:

a densified annular porous structure having a first carbon matrixdeposited within the annular porous structure and a second carbon matrixdeposited within the annular porous structure overlying the first carbonmatrix, the densified annular porous structure having two generallyparallel planar surfaces bounded by an inside circumferential surfaceand an outside circumferential surface spaced from and encircling theinside circumferential surface, a first circumferential portion adjacentthe inside circumferential surface, and a second circumferential portionadjacent the outside circumferential surface, wherein the first andsecond circumferential portions are bounded by the two generallyparallel planar surfaces, the second circumferential portion having atleast 10% less of the first carbon matrix per unit volume relative tothe first circumferential portion, the first carbon matrix and thesecond carbon matrix having a substantially rough laminarmicrostructure, and the first carbon matrix being more graphitized thanthe second carbon matrix.

According to still another aspect of the invention, a CVI/CVD process ina CVI/CVD furnace is provided, comprising the steps of:

introducing a reactant gas into a sealed preheater disposed within theCVI/CVD furnace, the sealed preheater having a preheater inlet and apreheater outlet, the reactant gas being introduced into the preheaterinlet and exiting the sealed preheater through the preheater outlet andinfiltrating at least one porous structure disposed within the CVI/CVDfurnace;

heating the at least one porous structure;

heating the sealed preheater to a preheater temperature greater than thereactant gas temperature;

sensing a gas temperature of the reactant gas proximate the outlet;

adjusting the preheater temperature to achieve a desired gastemperature; and,

exhausting the reactant gas from the CVI/CVD furnace.

According to still another aspect of the invention, an apparatus isprovided for introducing a first reactant gas into a CVI/CVD furnace,comprising:

a first main gas line for supplying the first reactant gas;

a plurality of furnace supply lines in fluid communication with thefirst main gas line and the CVI/CVD furnace;

a plurality of first flow meters that measure a quantity of firstreactant gas flow through each furnace supply line; and,

a plurality of first control valves configured to control the quantityof flow of the first reactant gas through each furnace supply line.

According to still another aspect of the invention, a CVI/CVDdensification process is provided, comprising the steps of:

densifying a first porous wall within a CVI/CVD furnace by a pressuregradient CVI/CVD process wherein a first flow of reactant gas is forcedto disperse through the first porous wall;

densifying a second porous wall by a pressure gradient CVI/CVD processwherein a second flow of reactant gas is forced to disperse through thesecond porous wall; and,

independently controlling the first flow of the reactant gas and thesecond flow of the reactant gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a schematic sectional view of a CVI/CVD furnaceaccording to an aspect of the invention.

FIG. 2 presents a sectional view of a fixture according for a pressuregradient CVI/CVD process according to an aspect of the invention.

FIG. 3 presents a sectional view of a fixture according to an aspect ofthe invention.

FIG. 4 presents a sectional view of a fixture according to an aspect ofthe invention.

FIG. 5 presents a sectional view of a fixture according to an aspect ofthe invention.

FIG. 6 presents a sectional view of a fixture according to an aspect ofthe invention.

FIG. 7 presents a sectional view of a fixture according to an aspect ofthe invention.

FIG. 8 presents a sectional view of a densified structure according toan aspect of the invention.

FIG. 9 presents a sectional view of a densified structure according toan aspect of the invention.

FIG. 10 presents a sectional view of a densified structure according toan aspect of the invention.

FIG. 11 presents a sectional view of a densified structure according toan aspect of the invention.

FIG. 12 presents a sectional view of a densified structure according toan aspect of the invention.

FIG. 13 presents a sectional view of a densified structure according toan aspect of the invention.

FIG. 14 presents a sectional schematic view of a furnace for aconventional CVI/CVD process.

FIG. 15 presents a sectional schematic view of a furnace forsimultaneously densifying a large number of porous structures by apressure gradient CVI/CVD process according to an aspect of theinvention.

FIG. 16 presents a perspective view of a preheater according to anaspect of the invention.

FIG. 17 presents a fixture with porous structures according to an aspectof the invention.

FIG. 18 presents a fixture with porous structures according to an aspectof the invention.

FIG. 19 presents a process according to an aspect of the invention.

FIG. 20 presents a process according to an aspect of the invention.

FIG. 21 presents a process according to an aspect of the invention.

FIG. 22 presents an alternate cover plate for use with the preheater ofFIG. 16.

FIG. 23 presents a sectional view of a densified structure according toan aspect of the invention.

FIG. 24 presents a graph showing bulk density gain versus time for avariety of processes according to the invention.

FIG. 25 presents a graph showing average deposition rate versusnormalized reactant gas flow for a variety of processes according to theinvention.

FIG. 26 presents a graph showing average deposition rate versusnormalized reactant gas flow for a variety of reactor volume pressuresaccording to an aspect of the invention.

FIG. 27 presents a graph showing change in pressure across the porouswall versus average bulk density for a variety of reactant gas flowrates and reactor volume pressures according to an aspect of theinvention.

FIG. 28 presents a fixture for holding porous structures havingalternating “OD” and “ID” ring-like spacers.

FIG. 29 presents a fixture for holding porous structures having all “ID”ring-like spacers.

DETAILED DESCRIPTION OF THE INVENTION

The invention and various embodiments thereof are presented in FIGS. 1through 29 and the accompanying descriptions wherein like numbered itemsare identical. As used herein, the term “conventional CVI/CVD” isintended to refer to the previously described isothermal CVI/CVDprocess. The term “pressure gradient CVI/CVD” is intended to refer tothe previously described pressure gradient CVI/CVD or forced-flowprocess and is intended to specifically exclude the previously describedthermal gradient and thermal gradient-forced flow processes to theextent that these processes utilize an intentionally induced thermalgradient that affects the deposition process.

Referring now to FIG. 1, a schematic depiction is presented of a CVI/CVDfurnace 10 adapted to deposit a matrix within a porous structure 22 by apressure gradient CVI/CVD process according to an aspect of theinvention. The furnace 10 has a shell 13 with an inner surface 12 thatdefines a furnace volume 14, and a gas inlet 16 for introducing a gasinto the furnace 10. A susceptor 30 is disposed around the reactorvolume 35 and is induction heated by an induction coil 20 according tomethods well known in the art. Other methods of heating may also beutilized such as resistance heating and microwave heating, any of whichare considered to fall within the purview of the invention. Aninsulation barrier 31 is disposed between the susceptor 30 and theinduction coil 20. The susceptor 30 has an inner surface 33 that definesa reactor volume 35 which is included within the furnace volume 14. Theporous structure 22 is disposed within a fixture 2 in the reactor volume35 and is predominantly heated by radiation from the susceptor 30. Avacuum apparatus 58 comprising a vacuum pump or steam vacuum system isin fluid communication with an exhaust 32 and evacuates the furnacevolume 14 to a pressure below atmospheric pressure. A reactant gas isintroduced into the reactor volume 35 through the gas inlet 16 thatreceives the reactant gas from a furnace supply line 26. The reactantgas infiltrates the porous structure 22 where it cracks and deposits amatrix within the porous structure 22. A single type of gas or mixturesof multiple types of gases may be supplied to the gas inlet 16.

According to a preferred embodiment, the reactant gas comprises amixture of two reactant gases that are introduced through a first maingas line 42 and a second main gas line 44. The furnace supply line 26 isin fluid communication with the first and second main gas lines 42 and44 and the inlet 16 thereby serving to transfer the reactant gases tothe furnace 10. A first flow meter 46 measures the quantity of flow of afirst gas (indicated by arrow 50) introduced into the furnace supplyline 26 through the first main supply line 42, and a second flow meter48 measures the quantity of flow of a second gas (indicated by arrow 52)introduced into the furnace supply line 26 through the second main gasline 44. The flow of gas into furnace supply line 26 is controlled by afirst control valve 54 which controls the flow of the first reactant gasfrom the first main gas line 42, and by a second control valve 56 whichcontrols the flow of the second reactant gas from the second main gasline 44.

The porous structure 22 includes a porous structure aperture 23. A tube60 is in fluid communication with fixture 2 and the inlet 16 therebyserving to transfer the reactant gas to the fixture 2. The fixture 2comprises a pair of plates 38 and 40, and the tube 60 is sealed to thegas inlet 16 and to the plate 38. The porous structure 22 is sealedbetween the plates by ring-like spacers 62 and 64, and the plates 38 and40 are held together by tie-rods 66. The porous structure 22 forms aporous wall 68 disposed between the inlet 16 and the exhaust 32. Thefurnace volume 14 and reactor volume 35 are reduced to a pressure belowatmospheric pressure, and the gas is introduced to the porous structureaperture 23 at a greater pressure than the reactor volume pressure whichdevelops a pressure gradient through the porous wall 68 and forcesdispersion of the gas through porous structure 22 before being withdrawnfrom the reactor volume 35 and the furnace volume 14 by the vacuumapparatus 58 as indicated by arrows 34, 36, and 28.

Pressure inside the furnace volume is measured by an exhaust pressuresensor 72, and pressure inside the porous structure aperture 23 ismeasured by an inlet pressure sensor 70. An approximate reactant gastemperature inside the porous structure aperture 23 is measured by aflow temperature sensor 74, and porous structure temperature isapproximated by a structure temperature sensor 76 which is placed inclose proximity to the plate 40. As will be discussed in more detail,the temperature and pressure conditions are chosen to cause the gas tocrack and deposit a matrix having certain desired properties within theporous structure 22. The various aspects of the invention may be used todeposit any type of CVI/CVD deposited matrix including, but not limitedto, carbon or ceramic matrix deposited within carbon or ceramic basedporous structures 22. The invention is particularly useful fordepositing a carbon matrix within a carbon-based porous structure, andespecially for making carbon/carbon composite structures such asaircraft brake disks.

Referring now to FIG. 2, a detailed view of the fixture 2 for holdingporous structure 22 is presented. According to a preferred embodiment,the porous structure is annular and has two opposing generally planarsurfaces 78 and 80 that are bounded by an inside circumferential surface82 and an outside circumferential surface 84. An “OD” (outside diameter)type ring-like spacer 64 having a mean diameter less than the outsidecircumferential surface 84 is placed between the porous structure 22 andthe plate 38. An “ID” (inside diameter) type ring-like spacer 62 havinga mean diameter slightly greater than the inside circumferential surface82 is placed between the porous structure 22 and the plate 40. Thering-like spacers 62 and 64 also serve as spacers to permit gas flowbetween porous structure 22 and the plates 38 and 40, and also seal theporous structure 22 to the plates 38 and 40. The tie-rods 66 may bethreaded on one or both ends and include nuts 67 in threaded engagement.Washers 69 may be used to distribute the load to the plates 38 and 40.

As discussed previously, the furnace volume is subjected to a vacuum andreactant gas is introduced into the tube 60 at a greater pressure thanthe furnace volume. Thus, a first portion 86 (indicated by finecrosshatching) of the fibrous structure 22 is subjected to a greaterpressure than a second portion 88 (indicated by fine crosshatching) ofthe fibrous structure 22 which induces dispersion of the reactant gasthrough the porous structure 22 as indicated by the arrows 90. As gasdisperses through the porous structure, additional gas flows through thetube 60 and toward the porous structure 22 as indicated by arrows 92.Thus, reactant gas is continuously supplied and forced to dispersethrough the porous structure 22. In this example, the first portion 86includes one surface 78 of the two opposing surfaces 78 and 80, and thesecond portion 88 includes the other surface 80 of the two opposingsurfaces 78 and 80. The first portion 86 also includes the insidecircumferential surface 82, and the second portion 88 includes theoutside circumferential surface 84.

Referring now to FIG. 3, an alternative fixture 4 that may be used inplace of fixture 2 is depicted wherein two porous structures 22 arestacked and simultaneously densified. Two ring-like spacers 64 areutilized and tie-rods 65 are longer versions of the tie-rods 66 of FIG.2. A pressure gradient is applied to the porous structure (as previouslydescribed in relation to FIG. 2) resulting in dispersion of the reactantgas through the porous structures 22 as indicated by arrows 90. Otherfeatures of fixture 4 are identical to fixture 2.

The reactant gas tends to crack and preferentially deposit the matrixwithin the portions of the porous structure 22 subjected to a pressurerelatively greater than the pressure in other portions. For example,FIG. 8 presents a densified structure 300 that results from the FIGS. 2and 3 processes beginning with a porous structure 22. The degree ofcrosshatching is intended to generally indicate relative density: finelycrosshatched areas represent greater density relative to coarselycrosshatched areas. The density monotonically decreases from a greatestdensity zone 302 to a least density zone 308 with density zones 304 and306 representing intermediate density ranges. The densified structure300 has an average bulk density, and density zone 302 is typically110%-140% of the average bulk density, and density zone 308 is typically60%-90% of the average bulk density. Note that the highest density zone302 generally corresponds to the first portion 86 and the lowest densityzone 308 generally corresponds to the second portion 88. Thus, the firstportion 86 has a greater bulk density gain than the second portion 88during the pressure gradient CVI/CVD process depicted in FIGS. 2 and 3.

The density gradient depicted in FIG. 8 is unacceptable for manyapplications. The density gradient may be reduced by depositing a firstmatrix within the porous structure with a pressure gradient CVI/CVDprocess, as shown in FIGS. 2 and 3. In this first process, the firstportion 86 has a greater bulk density gain than the second portion 88,as shown in FIG. 8. Subsequently, the porous structure 22 may be furtherdensified by depositing a second matrix with at least one additionaldensification process in which the second portion 88 has a greater bulkdensity gain than the first portion 86. For example, the partiallydensified structure 300 of FIG. 8 could be flipped and subjected to thepressure gradient CVI/CVD process depicted in FIGS. 2 and 3. The secondportion 88 is subjected to a greater pressure than the first portion 86,which results in the second portion 88 having a greater bulk densitygain than the first portion 86. FIG. 9 depicts a densified structure 310resulting from this two-step/flip process. The density monotonicallydecreases from a greatest density zone 312 to a least density zone 316with density zone 314 representing an intermediate density range. Thedensified structure 310 has an average bulk density, and density zone312 is typically 105%-115% of the average bulk density, and density zone316 is typically 85%-95% of the average bulk density. The densitygradient is now generally symmetrical through the thickness of theporous structure 22 which is desirable for brake disk applications. Thedensity gradient is also less than the density gradient of the densifiedstructure 300 depicted in FIG. 8. The second or additional densificationprocesses may include pressure gradient CVI/CVD, conventional CVI/CVD,and resin impregnation followed by charring. In addition, a porousstructure partially densified with a carbon matrix may be heat treatedat a temperature greater than the processing temperatures of previousCVI/CVD processes to increase graphitization of the carbon matrix beforefurther depositing additional matrix.

Referring now to FIG. 4, another alternative fixture 6 that may be usedin place of fixture 2 for an alternative pressure gradient CVI/CVDprocess is presented. The fixture 6 utilizes all “ID” ring-like spacers62 resulting in only the inner circumferential surface 82 of each porousstructure being subjected to a greater pressure than the reactor volume35. Thus, a first portion 87 of porous structure 22 is subjected to agreater pressure than a second portion 89 resulting in pressure drivenflow of the reactant gas through the porous structures 22 as indicatedby arrows 91. In this example, the first portion 87 includes the insidecircumferential surface 82, and the second portion 89 includes theoutside circumferential surface 84 and two opposing surfaces 78 and 80.The reactant gas tends to quickly flow through the porous structure 22and exit near the ring-like spacer 62. Thus, reactant gas is not forcedto disperse through all of the porous structure 22. FIG. 10 presents adensified structure 320 generated by the FIG. 4 process. The densifiedstructure 320 comprises a zone 322 of greatest density adjacent theinside circumferential surface 82 which drops off to a zone 328 of leastdensity at the core. The density monotonically increases from the leastdensity zone 328 to the greatest density zone 322 with density zones 324and 326 representing intermediate density ranges. The densifiedstructure 320 has an average bulk density, and density zone 322 istypically about 140% of the average bulk density, and density zone 324is typically about 115% of the average bulk density. Density zone 328 istypically about 80% of the average bulk density. The zone 322 ofgreatest density generally corresponds with the first portion 87 of FIG.4. The region of intermediate density 324 adjacent the outsidecircumferential surface 320 is generated by a conventional CVI/CVDprocess induced by reactant gas flow that has not fully crackedexhausting from adjacent porous structures. The densified structure 320may be further densified by second or additional densification processeswhich include pressure gradient CVI/CVD, conventional CVI/CVD, and resinimpregnation followed by charring.

Referring now to FIG. 5, an alternative fixture 8 that may be used inplace of fixture 2 for an alternative pressure gradient CVI/CVD processis presented. The fixture 8 utilizes all “OD” ring-like spacers 64resulting in the inside circumferential surface 82 and the opposingsurfaces 78 and 80 of each porous structure being subjected to a greaterpressure than reactor volume 35. The outside circumferential surface 84is subjected to the pressure of the reactor volume 35. Thus, a firstportion 94 of porous structure 22 is subjected to a greater pressurethan a second portion 96 resulting in pressure driven flow of thereactant gas through the porous structures 22 as indicated by arrows 98.In this example, the first portion 94 includes the insidecircumferential surface 82 and the opposing surfaces 78 and 80, and thesecond portion 96 includes the outside circumferential surface 84. Asdepicted, the reactant gas is forced to disperse through all of theporous structure 22. FIG. 11 presents a densified structure 330generated by the FIG. 5 process. The densified structure 330 comprises azone 332 of greatest density adjacent the inside circumferential surface82 and part of the two opposing surfaces 78 and 80. The zone 332sometimes extends all the way to the outside circumferential surface 84and includes essentially all of the opposing surfaces 78 and 80. Thedensity monotonically decreases from the greatest density zone 332 to aleast density zone 338 with density zones 334 and 336 representingintermediate density ranges. The densified structure 330 has an averagebulk density, and density zone 332 is typically 110%-125% of the averagebulk density, and density zone 338 is typically 80%-90% of the averagebulk density. The FIG. 5 process generates a densified structure 330that has a symmetric density gradient through the structure thickness.However, the density gradient may be skewed toward one of the surfaces78 or 80 in some densified structures 330 due to process variations.Note that the zones 332 and 334 generally correspond to the firstportion 94 of FIG. 5, and the second portion 96 experiences a relativelyless bulk density gain as indicated by zones 336 and 338. The densifiedstructure 330 may be further densified by second or additionaldensification processes which may comprise pressure gradient CVI/CVD,conventional CVI/CVD, or resin impregnation followed by charring.

Referring now to FIG. 12, a densified structure 340 is presented that isgenerated by further densifying the porous structure 330 of FIG. 11 by aconventional CVI/CVD process. As shown, the greatest density appears ina zone 342 adjacent the inside circumferential surface 82, which isresidual from zone 332 of FIG. 11. The subsequent conventional CVI/CVDprocess decreases the radial density gradient. This is indicated by azone 344 of intermediate density adjacent the outside circumferentialsurface 84. A zone of lesser density 346 encircles a core zone 348 ofleast density. The subsequent process fills the lower density portionsremaining in the densified structure 330 of FIG. 11. Thus, the secondportion 96 from the FIG. 5 process experiences a greater bulk densitygain than the first portion 94 during the subsequent conventionalCVI/CVD process. In addition, the pressure gradient CVI/CVD processgenerated by the FIG. 5 process produces a desirable porositydistribution in densified structure 330 that renders structure 330extremely susceptible to subsequent densification by conventionalCVI/CVD processes. Densified structure 330 reaches final density quickerand has minimal tendency to seal-coat during subsequent conventionalCVI/CVD processes than a structure having the same bulk density that waspreviously densified by only conventional CVI/CVD processes. Thisgreatly minimizes the need for surface machining operations during thesubsequent processes, which greatly simplifies and expedites the entiredensification process. This synergistic effect was a surprisingdiscovery.

Referring now to FIG. 6, an alternative fixture 9 that may be used inplace of fixture 2 for an alternative pressure gradient CVI/CVD processis presented. The process presented in FIG. 6 is a “reverse flow”process wherein the reactant gas enters the porous structure 22 from theoutside rather than the inside of the porous structure 22. This isaccomplished by disposing the porous structure 22 between plates 38 and41. Plate 41 is essentially identical to plate 40 except that plate 41includes an aperture 43. A cylindrical barrier structure 350 is disposedbetween and sealed to plates 38 and 41. The barrier structure 350encircles the porous structure 22. The outside diameter of surface 80 isspaced from and sealed to the plate 41 by an “OD” ring-like spacer 64.The outside diameter of surface 78 is spaced from and sealed by an “OD”ring-like spacer 64 to a seal plate 352, which is disposed between theporous structure 22 and plate 38. A plurality of spacing blocks 353space the seal plate 352 from the plate 38 thereby forming a pluralityof apertures 354. Reactant gas is introduced into fixture 9 thedirection of arrow 92. The seal plate 352 forces the gas to flowradially outward and through the apertures 354. The barrier structure350 then forces the gas to flow upward as indicated by arrows 356 towardthe outside circumferential surface 84 of porous structure 22. Theaperture 43 in plate 41 subjects the inside of the fixture to thefurnace volume pressure which is less than the gas supply pressure intube 60. Thus, a first portion 95 is subjected to a greater pressurethan a second portion 97 which forces the gas to disperse through theporous structure 22 as indicated by arrows 99. The gas exhausts fromfixture 9 to the reactor volume 35 through the aperture 43 as indicatedby arrow 358. In this example, the first portion 95 includes the outsidecircumferential surface 84, and the second portion 97 includes theinside circumferential surface 82 and the opposing surfaces 78 and 80.The densified structure may be further densified by second or additionaldensification processes including pressure gradient CVI/CVD,conventional CVI/CVD, or resin impregnation followed by charring.

Referring now to FIG. 7, an alternative fixture 7 that may be used inplace of fixture 2 for an alternative pressure gradient CVI/CVD processis presented. FIG. 7 presents a reverse flow process which is verysimilar to the FIG. 6 process. Fixture 7 is essentially identical tofixture 9, except that fixture 7 comprises “ID” ring-like spacers 62rather than “OD” ring like spacers 64. The flow of reactant gas entersthe opposing surfaces 78 and 80 and the outside circumferential surface84, and exits the inside circumferential surface 82 of porous structure22 as indicated by arrows 101. The inside circumferential surface 82 issubjected to the pressure of the reactor volume 35, and the outsidecircumferential surface 84 and the opposing surfaces 78 and 80 aresubjected to the reactant gas supply pressure. Thus, a first portion 552of porous structure 22 is subjected to a greater pressure than a secondportion 550. In this example, the first portion 552 includes the insidecircumferential surface 82, and the second portion 550 includes theoutside circumferential surface 84 and the opposing surface 78 and 80.FIG. 13 presents a densified structure 341 generated by the FIG. 7process. The densified structure 341 comprises a zone 343 of greatestdensity adjacent the outside circumferential surface 84 and part of thetwo opposing surfaces 78 and 80. The density monotonically decreasesfrom the greatest density zone 343 to a least density zone 349 withdensity zones 345 and 347 representing intermediate density ranges. Thedensified structure 341 has an average bulk density, and density zone343 is typically about 120% of the average bulk density, and densityzone 349 is typically about 80% of the average bulk density. Thedensified structure 341 may be further densified by second or additionaldensification processes including pressure gradient CVI/CVD,conventional CVI/CVD, or resin impregnation followed by charring.

The various components of fixtures 2, 4, 6, 7, 8 and 9 are preferablyformed from graphite, but any suitable high temperature resistantmaterial may be used in the practice of the invention. The variousjoints may be sealed using compliant gaskets and/or liquid adhesivessuch as a graphite cement. The porous structures may be pressed againstthe ring-like spacers to form appropriate seals if the porous structuresare compliant before densification. Suitable compliant gaskets may beformed from a flexible graphite such as EGC Thermafoil® brand flexiblegraphite sheet and ribbon-pack available from EGC EnterprisesIncorporated, Mentor, Ohio, U.S.A. Comparable materials are availablefrom UCAR Carbon Company Inc., Cleveland, Ohio, U.S.A.

A conventional CVI/CVD process may be carried out using a CVI/CVDfurnace 11 as depicted in FIG. 14. Furnace 11 is very similar to Furnace10 (see FIG. 1). However the fixture 2 is eliminated and replaced with afixture 360. Fixture 360 comprises a support plate 362 disposed on aplurality of support posts 364. The porous structure is disposed on aplurality of spacers 368 that separate the porous structure 22 from theplate 362 permitting dispersion of the reactant gas between the plate362 and the porous structure 22. The support plate 362 has a multitudeof perforations (not shown) to permit dispersion of reactant gas throughthe plate and around the porous structure 22. The support posts 364,spacers 368, and perforated support plate 362 are preferably formed fromgraphite. Tube 60 of FIG. 1 is replaced by a larger diameter tube 366.Gas enters the furnace volume and freely expands as indicated by arrows370. The gas passes over the porous structure as indicated by arrows 34and exhausts from the furnace volume 14 to the vacuum device 58 asindicated by arrows 36 and 28. Normally, only one temperature sensor 76is used which generally senses the temperature of porous structure 22.The pressure measured by pressure sensor 70 is only slightly greaterthan the pressure measured by pressure sensor 72 during a conventionalCVI/CVD process. A mixture of reactant gases may be introduced from mainsupply lines 42 and 44, as previously described in relation to FIG. 1.

With each of the FIG. 2 through FIG. 7 fixtures, each annular porousstructure 22 has a surface area with a majority (more than 50%) of thesurface area of each annular porous structure being exposed to thereactant gas as it enters or leaves the porous structure 22.Establishing a high level of exposure reduces the pressure gradientrequired to force dispersion of the gas through each porous structure.As much of the porous structure surface area as possible is preferablyexposed to the reactant gas. Preferably, at least 80% of the porousstructure surface area is exposed.

Referring now to FIG. 15, a CVI/CVD furnace 400 and an apparatus 402 forintroducing a first reactant gas into the furnace 400 is presented.Furnace 400 and apparatus 402 are particularly suited for simultaneouslydensifying large quantities of porous articles, for example five hundredto one thousand annular preforms for manufacturing aircraft brake disks.A first main gas line 404 supplies the first reactant gas as indicatedby arrow 406. A plurality of furnace supply lines 408 are in fluidcommunication with the first main gas line 404 and the CVI/CVD furnace400. A plurality of first flow meters 410 measures a quantity of firstreactant gas flow through each furnace supply line 408. A plurality offirst control valves 412 are configured to control the quantity of flowof the first reactant gas through each furnace supply line 408.Apparatus 402 comprises four supply lines 408, four control valves 412,and four flow meters 410, but the invention is not limited to four ofeach component, since the number may be increased or decreased asrequired.

According to a preferred embodiment, the furnace 400 and reactant gassupply apparatus 402 are controlled by a controller 414. Each flow meter410 may communicate the measured quantity of flow to the controller 414via a first flow sensor line 416, and the controller 414 may controleach control valve 412 via a first valve control line 418. Thus, thequantity of flow of the first reactant gas into the furnace 400 may beindependently set and controlled for each supply line 408. Thecontroller 414 is preferably micro-processor based and comprises ascreen 415 for monitoring the various conditions and control states inthe reactant gas supply apparatus 402 and the furnace 400. According toa certain embodiment, each furnace supply line 408 comprises one firstflow meter 410 and one first control valve 412, as shown in FIG. 15, anda first main control valve 420 disposed within the first main gas line404. The first main control valve 420 preferably controls pressure inthe first main gas line 404. A first main flow meter 422 may also bedisposed within the first main gas line 404.

A mixture of gases may be supplied to furnace 400 by providing at leasta second main gas supply line 424 for supplying a second reactant gas asindicated by arrow 426. A plurality of second flow meters 430 areprovided that measure a quantity of second reactant gas flow througheach furnace supply line 408 with a plurality of second control valves432 configured to control the quantity of flow of the second reactantgas through each furnace supply line 408. Each second flow meter 430 maycommunicate the measured quantity of flow to the controller 414 via asecond flow sensor line 436, and the controller 414 may control eachsecond control valve 432 via a second valve control line 438. Accordingto a certain embodiment, the second main gas line 424 comprises a secondmain control valve 440 disposed within the second main gas line 424. Asecond main flow meter 442 may also be disposed within the second maingas line 424. The second main control valve 440 preferably controlspressure in the second main gas line 424.

The furnace 400 comprises a furnace shell 444 that defines a furnacevolume 446. A reactor volume 447 is included within the furnace volume446. The furnace supply lines 408 are in fluid communication with thereactor volume 447. A vacuum apparatus 448 is in fluid communicationwith the furnace volume 446 and reactor volume 447 via exhaust stacks450. The vacuum apparatus 448 reduces the pressure in furnace volume 446to a vacuum pressure (below atmospheric) and may comprise any suitabledevice such as a vacuum pump or steam vacuum system with appropriatefilters and scrubbers that remove undesirable by-products from the spentreactant gas. The reactant gas from a given furnace supply line 408 isintroduced into a corresponding preheater 458. A first preheater 458 isdisposed within the reactor volume 447 and has an inlet 460 and anoutlet 461. The first preheater 458 is sealed such that substantiallyall of the reactant gas introduced into the inlet 460 from acorresponding furnace supply line 408 is heated and leaves the preheaterthrough the corresponding outlet 461 where it infiltrates at least oneporous structure disposed within the furnace. The term “substantiallyall of the gas” is intended to allow for a small amount of leakage. Thefirst preheater 458 is heated to a preheater temperature greater thanthe reactant gas temperature from the corresponding furnace supply line408. The porous structure is also heated. In this example the porousstructure comprises a first porous wall 452 disposed within the reactorvolume 447. The first porous wall 452 is preferably annular andcomprises a first top plate 454 that seals the upper open end of thefirst porous wall 452, thereby defining a first enclosed cavity 456. Theother end of the first porous wall 452 is sealed against the firstpreheater 458, with the first preheater outlet 461 in fluidcommunication with the first enclosed cavity 456.

A first flow of reactant gas is introduced into the first preheater 458,and then passes into the first enclosed cavity 456 at a pressure greaterthan the pressure within the reactor volume 447. Thus, one side of thefirst porous wall 452 is subjected to a greater reactant gas pressurethan the other side of the first porous wall. In the example shown inFIG. 15, the inner side of the porous wall 452 (the enclosed cavity 456)is subjected to a greater reactant gas pressure than the outer side ofporous wall 452. The pressure difference forces the first flow ofreactant gas to disperse through the first porous wall 452 where theheated gas cracks and deposits a binding matrix within the heated firstporous wall 452. The remaining gas and any by-products then exit thefirst porous wall 452 and are exhausted from the reactor volume 447through exhaust stacks 450 by vacuum apparatus 448. Thus, the reactantgas is forced to disperse through the annular porous wall by introducingthe reactant gas to the CVI/CVD furnace and exhausting the reactant gasfrom the CVI/CVD furnace on opposite sides of the annular porous wall.At least one exhaust stack 450 is preferably provided between each pairof porous walls. Also, each preheater 458 may supply reactant gas tomore than one annular porous wall 452. Furnace 400 may be heated by anymethod known in the art for heating a CVI/CVD furnace, includingresistance heating and induction heating.

According to a preferred embodiment, the preheater 458 and porous wall452 are radiation heated by a susceptor 462 that encloses the firstpreheater 458 and porous wall 452 on all sides. The susceptor 462defines the reactor volume 447 and a floor 463 upon which the firstpreheater 458 rests. The susceptor 462 preferably comprises acircumferential portion 464 and the furnace 400 comprises a firstinduction coil 466, a second induction coil 468, and a third inductioncoil 470 that encircle the circumferential portion 464. The susceptor462 is inductively coupled with the induction coils 466, 468, and 470which transfer energy to the susceptor 462, where it is transformed intoheat in a manner well known in the art. Maintaining a uniformtemperature from the bottom to the top of a CVI/CVD furnace duringdensification of a large number of porous structures (hundreds) may bedifficult. The rate at which the gas cracks and deposits the bindingmatrix is largely determined by temperature assuming the reactant gasconcentration is sufficient. Thus, variations in porous structuretemperature throughout the furnace cause corresponding variations inbulk density gain which can reduce yield during a given CVI/CVD run.Incorporating multiple induction coils, as depicted in FIG. 15, permitsapplication of differing amounts of heat along the length of thefurnace. A more uniform porous structure temperature profile along thelength of the furnace (in direction of gas flow) may thus be obtained.

According to a further embodiment, a first gas temperature of the firstflow of reactant gas is sensed proximate the first preheater outlet 461by a first temperature sensor 490. Temperature sensor 490 may comprise aType K thermocouple with appropriate protective sheathing. The preheatertemperature may be adjusted to achieve a desired gas temperature.Measuring the preheater temperature directly is not necessary since thepreheater temperature is convectively related to the gas temperature atthe outlet 461. The preheater temperature is adjusted by increasing ordecreasing the heating of the first preheater 458. In FIG. 15, thesusceptor wall 464 is comprised of a first susceptor wall portion 467, asecond susceptor wall portion 469, and a third susceptor wall portion471. As previously described, the first induction coil 466 isinductively coupled to the first susceptor wall portion 467 in a mannerthat transforms electrical energy from the first induction coil 466 toheat energy in the first susceptor wall portion 467. The same applies tothe second susceptor wall portion 469 and the second induction coil 468,and the third susceptor wall portion 471 and third induction coil 470.The first preheater 458 is predominantly heated by radiation heat energyfrom the first susceptor wall portion 467 which is adjacent the firstinduction coil 466. Thus, the first preheater temperature may beadjusted by adjusting electrical power to the first induction coil 466.The electrical power to the second induction coil 468 and 470 may beadjusted as necessary to maintain a desirable porous structuretemperature profile along the length of the furnace. The first preheater458 is preferably disposed proximate the first susceptor wall portion467 which improves the transfer of heat energy by radiation. Thetemperature sensed by first temperature sensor 490 may be transmitted tothe controller 414 via a first temperature sensor line 494. Thecontroller may process the temperature sensor information andautomatically adjust electrical power to the first induction coil 466 asnecessary to achieve a desired temperature of the first gas flow as itleaves the first preheater outlet 461. In certain furnace arrangements,a preheater may be disposed proximate the center of the furnace andsurrounded by adjacent preheaters that are proximate the susceptor walland block transfer of heat energy by radiation to the center preheater.In such a case, the center preheater is heated predominantly byconduction from the adjacent preheaters that are heated by radiation.Thus, the center preheater is indirectly heated by radiation from thesusceptor wall and the center preheater temperature may be controlled byvarying power to the first induction coil 466. Also, the preheaterscould be resistance heated which would permit direct control of the heatenergy supplied to each preheater. Any such variations are considered tobe within the purview of the invention.

A second porous wall 472 may be sealed to a second preheater 478 withthe second porous wall having a second top plate 474. The secondpreheater 478 has a second preheater inlet 480 and a second preheateroutlet 481. A second temperature sensor 492 may be provided for sensingthe temperature of the second flow of reactant gas as it exits thesecond preheater outlet 481. The second porous wall 472 defines a secondenclosed cavity 476 that is in fluid communication with the secondpreheater outlet 481. A second flow of gas is introduced to the secondpreheater through a corresponding furnace supply line 408 and is forcedto disperse through the second porous wall 472 and exit the furnacevolume 446 in the same manner as described in relation to the firstporous wall 452. Thus, one side of the second porous wall 472 issubjected to a greater pressure than the other side of the second porouswall. According to a certain embodiment, the second preheater 478 andsecond porous wall 472 are heated predominantly by radiation from thesusceptor wall 464. The second preheater 478 is heated to a preheatertemperature greater than the reactant gas temperature from thecorresponding furnace supply line 408. The heated gas infiltrates thesecond porous wall 472 where it cracks and deposits a binding matrix.The remaining gas and any by-products then exit the second porous wall472 and are drawn out of the furnace volume 446 by vacuum apparatus 448.A second temperature sensor 492 may be provided proximate the secondpreheater outlet 481. The temperature sensed by second temperaturesensor 492 may be transmitted to the controller 414 via a secondtemperature sensor line 496. The controller 414 may process thetemperature sensor information and automatically adjust electrical powerto the first induction coil 466 as necessary to achieve a desiredtemperature of the second gas flow as it leaves the second preheateroutlet 481. Electrical power to the first induction coil 466 may also bemanually adjusted as necessary in order to achieve the desired gas flowtemperature. At least a third porous wall may be densified by a similarpressure gradient CVI/CVD process wherein at least a third flow ofreactant gas is forced to disperse through at least the third porouswall by subjecting one side of at least the third porous wall to agreater pressure than the other side of at least the third porous wall,and the third flow of gas may be independently controlled. Additionalporous walls may be added and densified in an identical manner usingadditional furnace supply lines 408 and additional preheaters.Additional preheaters and temperature sensors for sensing temperature ofthe gas flow proximate the outlet of each additional preheater may beprovided as required. Thus, the invention permits simultaneousdensification of large numbers of porous walls.

A porous wall temperature sensor 498 may be provided in close proximityto the first porous wall 452 for sensing a first porous walltemperature. The first porous wall temperature may be increased ordecreased by increasing or decreasing the first flow of reactant gasthat passes through the first porous wall 452. For example, the firstflow of reactant gas may be at a lesser temperature than the porousstructure as it exits the first preheater outlet 461. Increasing thefirst flow of reactant gas at this lesser temperature tends to decreasethe porous wall temperature and decreasing the flow tends to increasethe porous wall temperature. The reverse would apply if the first flowof reactant gas was at a greater temperature than the first porous wall452. The first porous wall temperature sensor 498 may communicate withthe controller 414 via a first porous wall temperature sensor line 502which permits automatic or manual control of the first gas flow asnecessary to achieve a desired first porous wall temperature. A secondporous wall temperature may be similarly sensed by a second porous walltemperature sensor 500. The second porous wall temperature sensor 500may communicate with the controller 414 via a second porous walltemperature sensor line 504 which permits automatic or manual control ofthe second gas flow as necessary to achieve a desired second porous walltemperature by increasing or decreasing the second gas flow. Temperatureof third and additional porous walls may be sensed and controlled insimilar manner. Each individual flow of gas from the furnace supplylines 408 may be independently controlled in order to influence theCVI/CVD deposition process by virtue of the reactant gas supplyapparatus 402. The porous wall temperature sensors may also be inserteddirectly in to the porous walls as indicated by temperature sensor 506.A thermocouple may be placed between an adjacent pair of annular porousstructures if the porous wall is formed from a stack of porousstructures. Porous wall temperature may also be sensed by an opticalpyrometer 548 focused through a window 546 on an optical target 544disposed between an adjacent pair of porous walls 452 and 472.

According to a preferred embodiment, the furnace volume 446 ismaintained at a constant vacuum pressure. The pressure inside the firstenclosed cavity 456, second enclosed cavity 476, and any third oradditional enclosed cavities is determined by the flow of reactant gasintroduced into that cavity and the porosity of the corresponding porouswall. For example, the flow into the first enclosed cavity 456 may bemaintained at a constant value. At the beginning of the densificationprocess, the pressure inside the first enclosed cavity may be onlyslightly higher than the furnace volume pressure outside the enclosedcavity. The pressure inside the first enclosed cavity 456 increases asmatrix is deposited within the first porous wall 452 because porositydecreases and the quantity of first flow of reactant gas is constant.The pressure inside the first enclosed cavity 456 may be controlled byincreasing or decreasing the flow of reactant gas into the firstenclosed cavity. Increasing flow increases pressure and decreasing theflow decreases pressure. A first pressure sensor 508 may be provided forsensing the pressure inside the first enclosed cavity 456. The firstpressure sensor 508 may communicate via first pressure sensor line 512with the controller 414 which allows automatic or manual control of thequantity of flow introduced into the first enclosed cavity 456 asnecessary to achieve a desired pressure. A second pressure sensor 510and second pressure sensor line 514 may be provided for controlling theflow and pressure inside the second enclosed cavity 476 in like manner.Third and additional pressure sensors and pressure sensor lines may beprovided as required. The quantity of gas flow into a given enclosedcavity is preferably fixed and the pressure allowed to naturally rise asthe porous wall densities unless the pressure rises too rapidly orexceeds a maximum desired pressure, in which case the flow may bereduced or completely stopped. The reactant gas supply apparatus 402allows independent control of the flow to each porous wall. Monitoringthe pressure inside the porous cavity also provides a real timeindication of the degree of densification of each porous wall. The lackof a pressure rise, or an unusually slow pressure rise, indicates thepresence of a leak in the preheater and/or the porous wall. The processmay be terminated and subsequently restarted once the leak is locatedand fixed. An unusually rapid pressure may indicate sooting or tarringof one or more of the annular porous walls.

Referring now to FIG. 16, a preheater 100 is presented which is apreferred embodiment for the preheaters 458 and 478 of FIG. 15. Thepreheater 100 is described in more detail in a copending United Statespatent application entitled APPARATUS FOR USE WITH CVI/CVD PROCESSES,filed the same day as the present application naming James W. Rudolph,Mark J. Purdy, and Lowell D. Bok as inventors, and which is fullyincorporated herein by reference. The preheater 100 comprises a sealedduct structure 102 disposed within the furnace 10 and resting on thesusceptor floor 463. The preheater 100 receives gas from the gas inlet460 (FIG. 15). The gas inlet 460 may be connected to one or moreperforated tubes 19 which promote dispersion of the gas throughout thesealed duct structure 102. Preheater 100 comprises a sealed bafflestructure 108 that rests upon a sealed duct structure 102. The sealedbaffle structure 108 comprises an array of spaced perforated plates 128and 129 with a bottom perforated plate comprising a baffle structureinlet 104 and a top perforated plate comprising a baffle structureoutlet 106. The sealed duct structure 102 and sealed baffle structure108 are sealed to each other, and the sealed duct structure 102 issealed to the susceptor floor 463 at joint 118 so that gas cannot avoidflowing through the sealed baffle structure 108. The sealed ductstructure 102 comprises at least two pieces 119, 120, and 121, upperring 122 and lower ring 123 which together form several sealed joints124, 125, 166, 168, 170, 172, and 174. The support bars 119, 120, and121, and lower ring 123 support the weight of the sealed bafflestructure 108. A cover plate 110 preferably adjoins the sealed ductstructure 102 disposed over the baffle structure outlet 106. The coverplate 110 serves to support the porous structure fixtures. Cover plate110 is adapted for use with a pressure gradient CVI/CVD process andcomprises a plurality of apertures 114 and 116 with each apertureproviding reactant gas to an annular porous wall. The cover plate 110 issealed to the sealed duct structure 102 by a compliant gasket placed inthe joint between the sealed duct structure 102 and the cover plate 110.The perforated plates 128 and 129 are coterminous and arranged in astack that defines a baffle structure perimeter 132. Each sealed bafflestructure plate 128 comprises an array of perforations 130, with thearray of perforations 130 of one susceptor plate 128 being misalignedwith the array of perforations 131 of an adjacent susceptor plate 129.This arrangement greatly facilitates transfer of heat by radiation fromthe susceptor wall 464 directly to the perforated plates 128 and 129.The heat is transferred by conduction along plates 128 and 129 and tothe gas by forced convection. The baffle structure perimeter 132 issealed by compliant gaskets 134 and comprises the outer plane-wise limitof each susceptor plate 128 and 129 and is disposed in close proximityto the susceptor wall 464. The compliant gaskets 134 also serve to spacethe perforated plates 128 and 129 from each other. The sealed ductstructure 102 preferably defines a ledge 136 upon which said sealedbaffle structure 108 rests. In the embodiment presented, the supportbars 119, 120, and 121 define the ledge in combination with lower ring123. A plurality of posts 140 may be provided that facilitate loadingthe baffle structure 108 into the furnace and also further support thesealed baffle structure 108 and cover plate 110. Each post 140 comprisesan enlarged portion that defines a seat (not shown) which rests on thesusceptor floor 463. The sealed baffle structure 108 rests upon theseat. The various components of preheater 100 are preferably formed frommonolithic graphite. The various sealed joints are preferably formedusing compliant gaskets and/or graphite cement. Suitable compliantgaskets may be formed from a flexible graphite such as EGC Thermafoil®and Thermabraid® brand flexible graphite sheet and ribbon-pack availablefrom EGC Enterprises Incorporated, Mentor, Ohio, U.S.A. Comparablematerials are available from UCAR Carbon Company Inc., Cleveland, Ohio,U.S.A.

The porous walls 452 and 472 of FIG. 15 may be formed from stacks ofannular porous structures, which is particularly preferred formanufacturing aircraft brake disks. Referring now to FIG. 17, apreferred fixture 200 is presented for densifying a stack of annularporous structures 22 by a pressure gradient CVI/CVD process. The fixture200 is described in more detail in a copending United States patentapplication entitled APPARATUS FOR USE WITH CVI/CVD PROCESSES, filed thesame day as the present application naming James W. Rudolph, Mark J.Purdy, and Lowell D. Bok as inventors. Fixture 200 is preferably usedwith the preheater 100 of FIG. 16. The porous structures 22 are arrangedin a stack 202. The fixture comprises a base plate 204, a spacingstructure 206, and a top plate 208. The top plate 208 optionally has anaperture 210 which is sealed by a cover plate 212, compliant gasket 213,and weight 214. The base plate 204 is adapted to engage the cover plate110 and has a base plate aperture (item 216 in FIG. 18) that aligns withone of the cover plate apertures 114 or 116. The base plate 204 ispreferably located by a plurality of conical pins 226. The base plate204 has mating conical base plate holes that are aligned with andreceive the conical pins 226. This arrangement facilitates aligning thebase plate aperture with a corresponding cover plate aperture. The baseplate 204 is preferably sealed to the cover plate 110 by use of acompliant gasket.

The top plate 208 is spaced from and faces the base plate 204. Thespacing structure 206 is disposed between and engages the base plate 204and the top plate 208. In the embodiment presented, the spacingstructure comprises a plurality of spacing posts 218 disposed about thestack of porous structures and extending between the base plate 204 andthe top plate 208. Each post 218 has pins 220 at either end that arereceived in mating holes 224 in base plate 204 and top plate 208. Thespacing structure 206 preferably comprises at least three posts 218. Thespacing structure 206 could also be formed as a single piece, and otherarrangements for engaging the base plate 204 and top plate 208 arepossible, any of which are considered to be within the purview of theinvention. At least one ring-like spacer 234 is disposed within thestack 202 of porous structures 22 between each pair of neighboringporous structures 22. The ring-like spacer 234 encircles the neighboringporous structure apertures 23. At least one of the ring-like spacers 234is preferably disposed between the base plate 204 and porous structure22 adjacent the base plate 204, and between the top plate 208 and porousstructure 22 adjacent the top plate 208. The base plate 204, the stackof porous structures 202, and the at least one ring-like spacer 234define an enclosed cavity 236 extending from the base plate aperture(item 216 in FIG. 18), including each porous structure aperture 23, andterminating proximate the top plate 208. According to a certainembodiment, the outside diameter of ring-like spacer 234 is about 21.9inches and the spacer inside diameter is about 19.9 inches forprocessing annular porous structures 22 having an outside diameter ofabout 21 inches. The ring-like spacers are preferably at least 0.25 inchthick.

Referring to FIG. 18, a preferred fixture 201 is presented for pressuregradient CVI/CVD densifying simultaneously a large number of porousstructures 22. The spacing structure 207 comprises at least oneintermediate plate 272 disposed between the base plate 204 and the topplate 208 that divides the stack of porous structures 203. The posts 218extend between the top plate 208 and one of the intermediate plates 272,between the base plate 204 and another of the intermediate plates 272,and between adjacent pairs of intermediate plates 272. In otherrespects, fixture 201 is essentially identical to fixture 200. Eachintermediate plate 272 has an intermediate plate aperture 274therethrough is sandwiched between a pair of the porous structures 22.The enclosed cavity 236 further includes each intermediate plateaperture 274. At least one of the ring-like spacers 234 is disposed oneither side of and sealed to the intermediate plate 272 between theintermediate plate 272 and the porous structures 22. Multiple fixtures201 may be stacked. In such case, the base plate 204 from one fixture201 engages the top plate 208 of a lower fixture 201 with the upperfixture base plate aperture 216 in fluid communication with the lowerfixture top plate aperture 210. Thus, the enclosed cavity extends fromone fixture 201 to the next until being terminated by the cover plate212 disposed over the uppermost top plate aperture 210. As shown moreclearly in this view, the base plate 204 is provided with conical holes230 that receive a conical portion of the conical pins 226, and thecover plate 110 is provided with holes 228 that receive a cylindricalportion of the conical pins 226.

Referring now to FIG. 28, an alternative fixture 300 for pressuregradient densifying a stack of porous structures 302 is presented.Fixture 299 is essentially identical to fixture 200, except stack 302comprises “OD” (outside diameter) ring-like spacers 234 disposed aroundthe outside diameter of each porous structure 22 alternated with “ID”(inside diameter) ring-like spacers 284 disposed around the insidediameter of each porous structure. The OD ring-like spacers 234preferably have an inside diameter 233 slightly less than the porousstructure outside diameter 608, and an outside diameter 235 that isgenerally coterminous with the porous structure outside diameter 608.The ID ring-like spacers 284 preferably have an outside diameter 286slightly greater than the porous structure inside diameter 610, and aninside diameter 288 that is generally coterminous with the porousstructure inside diameter 610. With ID ring-like spacers 284, the porousstructure outside diameter 608 is greater than the outside diameter 286of the ring like spacer 284. The wall thickness of each ring-like spacer234 and 284 is preferably minimized in order to maximize exposure of theporous structure surface area to the reactant gas as it enters or leaveseach porous structure 22. Referring to FIG. 29, an alternative fixture301 for pressure gradient densifying a stack of porous structures 303 ispresented. Fixture 301 is essentially identical to fixture 200, exceptstack 303 comprises all “ID” ring-like spacers 284 disposed around theinside diameter of each porous structure.

The various components of fixtures 200, 201, 299 and 301 are preferablyformed from graphite. The various joints comprised within the fixturesare preferably sealed using compressible ring-like gaskets from aflexible graphite material, as previously disclosed. If the porousstructures 22 are compressible, they may be compressed directly againstthe ring-like spacers 234 to provide a sufficient seal and eliminate theneed for compressible gaskets between the porous structures 22 andring-like spacers 234. The ring-like spacers prior to use are preferablyseal-coated with a surface deposition of pyrolytic carbon whichfacilitates removal of the ring-like spacer from a densified porousstructure following deposition of the matrix.

Fixtures similar to fixtures 200 and 201 may be used in a conventionalCVI/CVD process in which the ring-like spacers 234 are replaced byspacer blocks that separate the porous structures and permit thereactant gas to freely pass through, over, and around the porousstructures 22. In such case, cover plate 110 may be replaced by coverplate 152 of FIG. 22 in order to promote dispersion of the reactant gasthroughout the furnace volume. Cover plate 152 comprises an array ofperforations 153. Sealing the various joints comprised within a fixtureadapted for a conventional CVI/CVD process is not necessary ordesirable.

Referring now to FIG. 19, a CVI/CVD process is presented according to anaspect of the invention. According to a preferred embodiment, amultitude of annular porous carbon structures are disposed within aCVI/CVD furnace such as furnace 400 (FIG. 15) using multiple fixturessuch as fixture 201 (FIG. 18) which are sealed to multiple preheaterssuch as preheater 100. Reactant gas is supplied to the furnace using anapparatus such as the gas supply apparatus 402 (FIG. 15). The furnace isheated until conditions are stabilized, and a first carbon matrix isdeposited within the porous structures by a pressure gradient CVI/CVDprocess. More support for the porous structures than depicted in FIGS.17 and 18 during the pressure gradient CVI/CVD process is not necessarysince the porous structures do not sag during the pressure gradientCVI/CVD process. The porous structures are then subjected to a heattreatment process without removing the porous structures from thefurnace or from the fixtures. Alternatively, the porous structures maybe removed from the furnace and pressure gradient CVI/CVD fixturesbefore the heat treatment process. The heat treatment process isconducted at a higher temperature than the previous deposition processtemperatures which increases graphitization of the first carbon matrix.Following heat treatment, the porous structures are then removed fromthe furnace and surface machined in order to derive an accurate bulkdensity measurement. Machining the surface may also increase openporosity at the surface. A second carbon matrix is then deposited withinthe porous structures by a conventional CVI/CVD process. Thus, thesecond matrix overlies the first matrix. After reaching final density,the densified structures are machined into final parts. In certainembodiment, the pressure gradient CVI/CVD process and conventionalCVI/CVD process are conducted at about 1750-1900° F., and heat treatmentis conducted at about 3300-4000° F. Thus, the first matrix has a greaterdegree of graphitization than the second matrix due to the intermediateheat treatment process.

Referring now to FIG. 20, an alternative process is presented thatbegins with a pressure gradient CVI/CVD process in which a first carbonmatrix is deposited within the porous structures. The porous structuresare then subjected to a heat treatment process without removing theporous structures from the furnace or from the fixtures. A second carbonmatrix is then deposited in another pressure gradient CVI/CVD processthat immediately follows the heat treatment process without removing theporous structures from the furnace or the fixtures. Alternatively, theporous structures may be removed from the furnace and pressure gradientCVI/CVD fixtures before the heat treatment process, and replaced in thepressure gradient CVI/CVD fixtures before the second pressure gradientCVI/CVD process. The porous structures are then subjected to a surfacemachining operation. Further second carbon matrix is then deposited in aconventional CVI/CVD process and the porous structures are machined intofinal parts. Leaving the porous structures in the same furnace andfixtures during the first and second pressure gradient processes and theheat treatment process results in a “continuous” process. Additionalsupport blocks between adjacent pairs of porous structures in thepressure gradient CVI/CVD fixtures may be necessary in order to preventsagging during the heat treatment process.

Referring now to FIG. 21, an alternative process is presented thatbegins with a pressure gradient CVI/CVD process in which a first carbonmatrix is deposited within the porous structures. The porous structuresare surface machined and a second carbon matrix is then deposited in aconventional CVI/CVD process followed by a heat treatment process. Afterheat treatment, the fully densified porous structures are then machinedinto final parts. It is evident that the sequences of the FIGS. 19-21processes may be rearranged, and additional steps inserted, withoutdeparting from the invention.

The first carbon matrix and second carbon matrix preferably comprise asubstantially rough laminar microstructure. A rough laminarmicrostructure has a greater density (about 2.1 g/cc), greater thermalconductivity, and lesser hardness than smooth laminar microstructure(1.9-2.0 g/cc or less). Rough laminar microstructure is particularlypreferred in certain carbon/carbon aircraft brake disks. Microstructuremay be optically characterized as described by M. L. Lieberman and H. O.Pierson, Effect of Gas Phase Conditions on Resultant Matrix Pyrocarbonsin Carbon/Carbon Composites, 12 Carbon 233-41 (1974).

Referring now to FIG. 23, a densified porous structure 600 manufacturedaccording to either the FIG. 19, 20 or 21 process is presented. Thedensified porous structure 600 comprises a first circumferential zone512 adjacent the inside circumferential surface 82, and a secondcircumferential zone 514 adjacent the outside circumferential surface84. The first and second circumferential zones 512 and 514 extend allthe way through the thickness of the densified porous structure 600 andare bounded by the opposing surfaces 78 and 80. Densified porousstructure 510 comprises a first carbon matrix deposited within a porousstructure comprised of carbon fibers according to a pressure gradientCVI/CVD process. According to a preferred embodiment, the first carbonmatrix is deposited by a process using fixtures 200 and/or 201 havingall “OD” ring-like spacers 234 (FIGS. 17 and 18) which is similar to theprocess described in relation to FIG. 5, resulting in the first carbonmatrix being deposited unevenly in a density distribution similar todensified porous structure 330 of FIG. 11. The first circumferentialzone 512 is subjected to a greater reactant gas pressure than the secondcircumferential zone 514 during the pressure gradient CVI/CVDdensification process which causes the first circumferential zone 512 toexperience a greater bulk density gain than the second circumferentialzone 514. According to a certain embodiment, the second circumferentialzone 514 has about 15% less of the first carbon matrix per unit volumerelative to the first circumferential zone 512, and the first carbonmatrix preferably has a substantially rough laminar microstructure. Thesecond circumferential zone 514 generally has at least 10% less of thefirst carbon matrix per unit volume relative to the firstcircumferential zone 512, and may have 20%, 30%, 40% or less of thefirst carbon matrix. Densified porous structure 510 also comprises asecond carbon matrix overlying the first carbon matrix that is depositedby a conventional CVI/CVD process resulting in the densified porousstructure 600 having a final density distribution similar to densifiedporous structure 340 of FIG. 12. The second carbon matrix alsopreferably has a substantially rough laminar microstructure. The firstand second carbon matrices preferably have at least 90% rough laminarmicrostructure, more preferably at least 95% rough laminarmicrostructure, and in certain preferred embodiments 100% rough laminarmicrostructure.

The first carbon matrix may be heat treated which causes the firstcarbon matrix to be more graphitized than the second carbon matrix.Increasing graphitization increases the apparent density and thermalconductivity. Thus, the original density gradient from the pressuregradient CVI/CVD process may be identified in the densified porousstructure 600 after deposition of the second carbon matrix. If the firstcarbon matrix has a distribution as shown in FIG. 11, the firstcircumferential portion 512 has a generally greater thermal conductivitythan the second circumferential portion 514, and a generally greaterapparent density than the second circumferential portion 514 even afterthe second carbon matrix is deposited. Closed porosity remaining withinthe densified porous structure 600 affects the measurement of apparentdensity. Porosity effects may be minimized by measuring apparent densityof crushed samples which will be referred to herein as crushed apparentdensity. According to a certain technique, crushed apparent density ismeasured by cutting a specimen from a densified porous structure andfracturing the specimen between parallel steel platens of a load testingmachine. The specimen is preferably fractured in a manner that maintainsthe specimen in one piece. This may be accomplished by compressing thesample past the yield point without fragmentation. Apparent density isthen measured according to the Archimedes technique using mineralspirits such as Isopar M (synthetic isoparaffinic hydrocarbon) availablefrom Exxon Chemical Americas, Houston, Tex., U.S.A. Vacuum is used toforce the mineral spirits into the structure. Apparent density is ameasurement of the density of the material that is impervious topenetration by the mineral spirits. Fracturing the specimen openspreviously closed porosity that was impervious to penetration by themineral spirits and minimizes porosity effects. Alternatively, crushedapparent density of a pulverized sample may be measured using a heliumpyconometer. Measurements of densified porous structures processedsimilar to densified porous structure 600 demonstrated that the crushedimpervious density adjacent the inside circumferential surface 82 wasconsistently at least 0.2% greater, and may be as much as 0.4% and 0.5%greater, than adjacent the outside circumferential surface 84. Thus,crushed apparent density tends to generally decrease from the insidesurface 82 to the outside surface 84.

Thermal conductivity of densified porous structures similar to densifiedporous structure 600 (as described in the immediately precedingparagraph) was measured in two directions: normal to the opposingsurfaces 78 and 80 which will be referred to as “thermal flatconductivity”, and normal to the circumferential surfaces 82 and 84 (inthe radial direction) which will be referred to as “thermal edgeconductivity.” Thermal flat conductivity of circumferential portion 514was at least 5% less than circumferential portion 512 when measured atthe opposing surfaces 78 and 80. Thermal flat conductivity ofcircumferential portion 514 was at least 12% less than circumferentialportion 512 at one-half of the distance between opposing surfaces 78 and80. Thermal edge conductivity of circumferential portion 514 was atleast 5% less than circumferential portion 512 when measured at theopposing surfaces 78 and 80. Thermal edge conductivity ofcircumferential portion 514 was at least 4% less than circumferentialportion 512 when measured at one-half of the distance between opposingsurfaces 78 and 80. Thus, thermal conductivity tends to generallydecrease from the inside circumferential portion 512 to the outsidecircumferential portion 514. This trend is induced by the first matrixbeing more graphitized than the second matrix.

The following examples further illustrate various aspects of theinvention.

EXAMPLE 1

A base-line was established for a conventional CVI/CVD process asfollows. A fibrous textile structure about 1.5 inch thick wasmanufactured according to FIGS. 1 through 4 of U.S. Pat. No. 4,790,052starting with a 320K tow of unidirectional polyacrylonitrile fiber. Anannular porous structure was then cut from the textile structure havingan outside diameter of about 7.5 inch, an inside diameter of about 2.5inch. The annular porous structure was then pyrolyzed to transform thefibers to carbon. The pyrolyzed porous structure, having a bulk densityof 0.49 g/cc, was then placed in a furnace similar to furnace 11 of FIG.14. Pressure was reduced to 10 torr inside the furnace volume and thefurnace was heated and stabilized at a temperature of about 1860° F.when measured by a temperature sensor positioned as temperature sensor76 of FIG. 14. A reactant gas mixture was introduced as described inrelation to FIG. 14 and allowed to freely disperse over and around theporous structure in a manner typical of a conventional CVI/CVD process.The reactant gas mixture was comprised of 87% (volume percent) naturalgas and 13% propane at a flow rate of 4000 sccm (standard cubiccentimeters per minute) and a residence time of about 1 second in thereactor volume. The natural gas had a composition of 96.4% methane(volume percent), 1.80% ethane, 0.50% propane, 0.15% butane, 0.05%pentane, 0.70% carbon dioxide, and 0.40% nitrogen. The process wasstopped three times to measure bulk density gain of the porousstructure. Total deposition process time was 306 hours. An average rateof deposition was calculated for each of the three densification runs.The test conditions and data from this example are presented in Table 1,including cumulative deposition time (Cum. Time) and total density gain(Density Gain) at each cumulative time noted. The carbon matrixdeposited within the densified porous structure at the end of theprocess comprised nearly all rough laminar microstructure with minimaldeposits of smooth laminar microstructure at the surface of the porousstructure.

TABLE 1 Cum. Gas Flow Part Density Time Rate Temp. Gain (hour) (sccm)(F.°) (g/cc)  41 4000 1857 0.310 166 4000 1860 0.886 306 4000 1855 1.101

EXAMPLE 2

An annular porous structure having a thickness of 1.6 inch, an outsidediameter of 6.2 inch, and an inside diameter of 1.4 inch was cut from afibrous textile structure and processed according to Example 1 by aconventional CVI/CVD process. The test conditions and data from thisexample are presented in Table 2.

TABLE 2 Cum. Gas Flow Part Density Time Rate Temp. Gain (hour) (sccm)(F.°) (g/cc) 92 4000 1858 0.370

EXAMPLE 3

Two annular porous structures (Disks A and B), prepared from a fibroustextile structure and having the same dimensions as described in Example1, were densified by a pressure gradient CVI/CVD process using a furnacesimilar to furnace 10 of FIG. 1, a fixture similar to fixture 2 of FIG.2 having ID/OD spacers, and the reactant gas mixture of Example 1. Thetest conditions and data from this example are presented in Table 3.Furnace pressure was 10 torr. Temperature of the gas stream wasestimated to be 1740° F. when measured by a temperature sensor such astemperature sensor 74 of FIG. 1. The gas was forced to flow through theporous structure, as previously described in relation to FIG. 2, at aflow rate of 4000 sccm. The carbon matrix deposited within Disk Acomprised all rough laminar microstructure. The microstructure of Disk Bwas not evaluated. Disk A was cut into smaller samples and the bulkdensity measurements of these samples were determined using theArchimedes method, and demonstrated a density profile similar to FIG. 8.

TABLE 3 Run Gas Flow Part Density Time Rate Temp. Gain Disk (Hour)(sccm) (F.°) (g/cc) A 165 4000 1861 1.106 B 123 4000 1859 0.928

EXAMPLE 4

Three annular porous structures (Disks A, B and C) were prepared andindividually densified by a pressure gradient CVI/CVD process accordingto Example 3 except that the porous structures were flipped part waythrough the process in order to obtain a more uniform final densitydistribution. Temperature of the gas stream was approximately 1740° F.when measured by a temperature sensor such as temperature sensor 74 ofFIG. 1. The test conditions and data from this example are presented inTable 4. The carbon matrix deposited within Disks A and C was all roughlaminar before the flip, and essentially smooth laminar after the flip.The microstructure of Disk B was not determined. The final densifiedporous structures had density profiles similar to FIG. 9.

TABLE 4 Cum. Gas Flow Part Density Time Rate Temp. Gain Disk (hour)(sccm) (F.°) (g/cc) A 72 4000 1859 0.743 Flip 96 4000 1859 0.853 111 4000 1855 1.034 B 49 4000 1854 0.619 Flip 74 4000 1849 0.898 C 49 40001858 0.625 Flip 75 4000 1853 0.915

EXAMPLE 5

Two annular porous structures, prepared from a fibrous textile structureand having the same dimensions as described in Example 1, weresimultaneously densified by a pressure gradient CVI/CVD process with afixture similar to fixture 6 of FIG. 4 having all “ID” spacers, and thereactant gas mixture of Example 1. Temperature of the gas stream wasestimated 1745° F. when measured by a temperature sensor such astemperature sensor 74 of FIG. 1. The test conditions and data from thisexample are presented in Table 5. Density gain on Table 5 is an averagefor the two disks. The carbon matrix deposited within the densifiedporous structure at the end of the process comprised all rough laminarmicrostructure. Computed tomagraphy scans of the disk demonstrateddensity profiles similar to FIG. 10.

TABLE 5 Cum. Gas Flow Part Density Time Rate Temp. Gain (hour) (sccm)(F.°) (g/cc) 24.4 8000 1860 0.262 70.7 8000 1856 0.593

EXAMPLE 6

Four annular porous structures, prepared from a fibrous textilestructure and having the same dimensions as described in Example 1, weredensified by a pressure gradient CVI/CVD process using a fixture similarto fixture 8 of FIG. 5 having all “OD” spacers, and the reactant gasmixture of Example 1. Two disks were simultaneously densified (Disk PairA and B) and reactant gas flow rate was doubled to maintain a flow rateof 4000 sccm per disk. Temperature of the gas stream was approximately1750° F. when measured by a temperature sensor such as temperaturesensor 74 of FIG. 1. The test conditions and data from this example arepresented in Table 6. The density gain on Table 6 is an average for eachdisk pair. The carbon matrix deposited within the densified porousstructure at the end of the process comprised all rough laminarmicrostructure. Computed tomagraphy scans of Disk Pair B demonstrateddensity profiles similar to FIG. 11.

TABLE 6 Run Gas Flow Part Density Disk Time Rate Temp. Gain Pair (hour)(sccm) (F.°) (g/cc) A 70 8000 1860 0.951 B 70 8000 1855 0.861

EXAMPLE 7

An annular porous structure was prepared from a fibrous textilestructure, having the same dimensions as described in Example 2, anddensified by a pressure gradient CVI/CVD process using a fixture similarto fixture 7 of FIG. 7 having all “ID” seals with a reverse flowprocess, and the reactant gas mixture of Example 1. Temperature of thegas stream was estimated 1730° F. when measured by a temperature sensorsuch as temperature sensor 74 of FIG. 1. The reactant gas was forced toflow through the porous structure as previously described in relation toFIG. 7 at a flow rate of 3000 sccm (the flow was lowered since the diskwas smaller than the disks used in Examples 3-6). The test conditionsand data from this example are presented in Table 7. The carbon matrixdeposited within the densified porous structure at the end of theprocess comprised mostly smooth laminar microstructure.

TABLE 7 Cum. Gas Flow Part Density Time Rate Temp. Gain (hour) (sccm)(F.°) (g/cc) 50 3000 1854 0.987

Referring now to FIG. 24, the data presented on Tables 1 through 7 isdepicted in graphical form. The data from Tables 1 and 2 is presented asa single smoothed curve 516 representing conventional CVI/CVD. The datafrom Tables 3 and 4 is presented as a single smoothed curve 518representing pressure gradient CVI/CVD using “ID/OD” spacers. The datafrom Table 5 is presented as a single smoothed curve 520 representingpressure gradient CVI/CVD using all “ID” spacers. The data from Table 6is presented as a single smoothed curve 522 representing pressuregradient CVI/CVD using all “OD” spacers. The data from Table 7 ispresented as curve 524 representing reverse flow pressure gradientCVI/CVD with all “ID” spacers. Densification rates increased by factorsfrom about one and one-half to five times conventional CVI/CVDdensification rates. Time to achieve a bulk density increase of 1 g/ccwas reduced by about 25% to 80% relative to conventional CVI/CVD time.The importance of eliminating as many leaks as possible is apparent fromFIG. 24. Any leakage tends to decrease the densification rate from themaximum attainable rate. Increased densification rates may be achievedeven with a small amount of leakage. Thus, some leakage may occur whileremaining within the purview of the invention.

Referring now to FIG. 25, curves representing densification rate versusnormalized flow are presented. The normalized flow is indicated as F*and represents a quantity of flow per unit of disk volume (for example,4000 sccm per 1000 cc disk volume=4 min⁻¹). Additional tests were runaccording to Example 6 and 7 above except flow rates of reactant gaswere varied from one test to the next. The data from tests conductedaccording to Example 6 with varying flow are presented on Tables 8, andthe data from tests conducted according to Example 7 with varying floware presented on Table 9. A curve 526 represents conventional CVI/CVD.Data from Table 8 is presented as curve 528 which represents pressuregradient CVI/CVD with all “OD” spacers (FIG. 5). Data from Table 9 ispresented as curve 530 which represents reverse flow pressure gradientCVI/CVD with all “ID” spacers (FIG. 7).

TABLE 8 Cum. Gas Flow Part Density Average Time Rate Temp. GainDeposition Rate (hour) (sccm) (F.°) (g/cc) (g/cc/h) 50 1000 1853 0.2320.0046 50 2000 1856 0.414 0.0083 50 4000 1851 0.547 0.0109 70 8000 18580.906 0.0129

TABLE 9 Cum. Gas Flow Part Density Average Time Rate Temp. GainDeposition Rate (hour) (sccm) (F.°) (g/cc) (g/cc/h) 50  500 1852 0.3230.0065 50 1000 1853 0.498 0.0100 56 2000 1855 0.920 0.0164 46 3000 18540.987 0.0215 38 4000 1852 0.919 0.0242

Referring now to FIG. 26, curves representing densification rate versusnormalized flow are presented. Additional tests were run according toExample 6 (pressure gradient with all “OD” spacers) above except thefurnace volume pressure and flow rates of reactant gas were varied fromone test to the next. The data from these tests is presented in Table10. Data from Table 10 is presented as three curves 532, 534, and 536.Curve 532 represents data at a furnace volume pressure of 10 torr whenmeasured by a pressure sensor such as sensor 72 of FIG. 1. Curve 534represents data at a furnace volume pressure of 25 torr when measured bya pressure sensor such as sensor 72 of FIG. 1. Curve 532 represents dataat a furnace volume pressure of 50 torr when measured by a pressuresensor such as sensor 72 of FIG. 1. The matrix deposited in all of thesetests comprised all rough laminar microstructure. As demonstrated byFIG. 26, additional gains in densification rate may be realized byincreasing the furnace volume pressure (Reactor Pressure) whilemaintaining a desired rough laminar microstructure. This was asurprising discovery.

TABLE 10 Average Cum. Reactor Gas Flow Part Density Deposition TimePressure Rate Temp. Gain Rate (hour) (torr) (sccm) (F.°) (g/cc) (g/cc/h)50 10 2000 1856 0.414 0.0083 50 10 4000 1851 0.547 0.0109 70 10 80001858 0.906 0.0129 50 25 2000 1853 0.449 0.0090 50 25 4000 1853 0.6110.0122 50 50 2000 1853 0.493 0.0099 50 50 4000 1852 0.683 0.0137

Referring now to FIG. 27, pressure differential across the porousstructure versus bulk density is presented for several reactant gas flowrates. Additional tests were run according to Example 6 with varyingflow rates. Data from these tests is presented in Table 11. The datafrom Table 11 is presented in FIG. 27 as a first set of curves 538 for aflow rate of 1000 sccm per disk, a second set of curves 540 for a flowrate of 2000 sccm per disk, and a third set of curves 542 for a flowrate of 4000 sccm per disk. The matrix deposited in all of these testscomprised all rough laminar microstructure. Table 11 includes theinitial pressure differential across the porous structures (Init. DeltaP), final pressure differential across the porous structures (FinalDelta P), and furnace volume pressure (Reactor Pressure) which wasmaintained constant. As demonstrated by FIG. 27, the pressure gradientacross the porous structure may be at least as high as 80 torr (whichindicates 90 torr on the high pressure side of the porous structure)while maintaining a desired rough laminar microstructure.

TABLE 11 Gas Average Init. Final Cum. Flow Density Deposit. Delta DeltaReactor Time Rate Temp Gain Rate P P Pressure (h.) (sccm) (° F.) (g/cc)(g/cc/h) (torr) (torr) (torr) 50 2000 1856 0.414 0.0083  5 1210 50 20001853 0.449 0.0090 10 025  50 2000 1853 0.493 0.0099  6 1650 50 4000 18510.547 0.0109 14 3210 50 4000 1853 0.611 0.0122 15 3925 50 4000 18520.683 0.0137 15 4250 70 8000 1860 0.951 0.0136 38 8110 70 8000 18550.861 0.0123 32 5610

Tests have demonstrated that the pressure gradient CVI/CVD processaccording to the invention may be conducted with a part temperature inthe range of 1800-2000° F., a reactor pressure in the range of 10-150torr, normalized reactant gas flow rate (F*) in the range of 0.4-10min¹, and with a hydrocarbon reactant gas mixture of natural gas and0-40% (volume percent) propane. Conducting the process within theseranges generally produces a rough laminar and/or smooth laminarmicrostructure. Conducting the process with all of these processparameters at or near the high extreme of each of these ranges mayresult in tarring or sooting. Other carbon bearing gases, pressures, andtemperatures known in the art for CVI/CVD processes may be substitutedwithout departing from the invention.

Densifying a porous structure by a pressure gradient CVI/CVD processaccording to the invention followed by a conventional CVI/CVD processproduces a densified porous structure having a more uniform densitydistribution than a comparable porous structure densified only by aconventional CVI/CVD process. According to a certain embodiment, forexample, an annular porous carbon structure having an inside diameter ofabout 10.5 inches (indicated as 602 in FIG. 23), a web (indicated as 604in FIG. 23) of about 5.25 inches, and a thickness (indicated as 606 inFIG. 23) of about 1.25 inches, is densified first with a carbon matrixdeposited by a pressure gradient CVI/CVD process (Example 6 conditions)using a fixture such as fixture 201 (FIG. 18) in furnace such as furnace400 (FIG. 15) resulting in a density distribution similar to densifiedstructure 330 of FIG. 11. Carbon matrix is further deposited by aconventional CVI/CVD process (Example 1 conditions) resulting in adensity distribution similar to densified structure 340 of FIG. 12, andhaving a mean bulk density of about 1.77 g/cc. According to standardstatistical practice, the standard deviation of the bulk densitythroughout the densified structure is about 0.06 g/cc. The standarddeviation of the bulk density throughout a comparable porous carbonstructure densified to an equivalent mean bulk density by onlyconventional CVI/CVD processes is about 0.09 g/cc. Thus, a porousstructure densified by a pressure gradient CVI/CVD process followed by aconventional CVI/CVD process is more uniform than a porous structuredensified by only conventional CVI/CVD processes. Circumferential aswell as total variation is reduced. Uniformity is desirable forcarbon/carbon aircraft brake disks.

The standard deviation of the bulk density throughout a carbon/carbonstructure manufactured according to the invention is preferably lessthan or equal to 0.07 g/cc, is more preferably less than or equal to0.06 g/cc or 0.05 g/cc, and is most preferably less than or equal to0.04 or 0.03 g/cc. Coefficient of variation of bulk density in anydensified porous structure is preferably less than or equal to 4%, morepreferably less than or equal 3.5% or 3%, and most preferably less thanor equal to 2.3% or 1.8%.

It is evident that many variations are possible without departing fromthe scope of the invention as defined by the claims that follow.

What is claimed is:
 1. A CVI/CVD process, comprising the steps of:partially densifying a porous structure within a CVI/CVD furnace bydepositing a first matrix within said porous structure with a pressuregradient CVI/CVD process in which a first portion of said porousstructure is subjected to a greater pressure than a second portion ofsaid porous structure and said first portion has a greater bulk densitygain than said second portion; and, subsequently densifying said porousstructure by depositing a second matrix within said porous structurewith at least one additional densification process in which said secondportion has a greater bulk density gain than said first portion.
 2. Theprocess of claim 1, wherein said additional densification process is aconventional CVI/CVD process.
 3. The process of claim 1, wherein saidadditional densification process is a pressure gradient CVI/CVD processin which said second portion is subjected to a higher pressure than saidfirst portion.
 4. The process of claim 1, wherein said additionaldensification process is a resin impregnation process, and furthercomprising the step of charring said resin.
 5. The process of claim 1,further comprising the step of heat treating said partially densifiedporous structure at a temperature greater than said pressure gradientCVI/CVD process before said step of subsequently densifying the porousstructure by at least one additional densification process.
 6. Theprocess of claim 1, wherein said porous structure is a carbon porousstructure and said pressure gradient CVI/CVD process deposits a carbonmatrix within said porous structure.
 7. The process of claim 1, whereinsaid porous structure is annular and has two generally planar opposingsurfaces, and said first portion includes one of said two opposingsurfaces, and said second portion includes the other of said twoopposing surfaces.
 8. The process of claim 1, wherein said porousstructure is annular and has an inside circumferential surface and anoutside circumferential surface, and said first portion includes saidinside circumferential surface, and said second portion includes saidoutside circumferential surface.
 9. The process of claim 1, wherein saidporous structure is annular and has an inside circumferential surfaceand an outside circumferential surface, and said first portion includessaid outside circumferential surface, and said second portion includessaid inside circumferential surface.
 10. The process of claim 1, whereinsaid porous structure is annular and has an inside circumferentialsurface and an outside circumferential surface, and said first portionincludes said inside circumferential surface, and said second portionincludes said outside circumferential surface.
 11. The process of claim1, wherein said porous structure is annular and has two generallyparallel planar surfaces bounded by an inside circumferential surfaceand an outside circumferential surface spaced from and encircling saidinside circumferential surface, and said first portion includes saidinside circumferential surface and one of said two generally parallelplanar surfaces, and said second portion includes said outsidecircumferential surface and the other of said two generally parallelplanar surfaces.
 12. The process of claim 1, wherein said porousstructure is annular and has two generally parallel planar surfacesbounded by an inside circumferential surface and an outsidecircumferential surface spaced from and encircling said insidecircumferential surface, and said first portion includes said outsidecircumferential surface and one of said two generally parallel planarsurfaces, and said second portion includes said inside circumferentialsurface and the other of said two generally parallel planar surfaces.13. The process of claim 1, further comprising the step of heat treatingsaid porous structure at a temperature greater than said pressuregradient CVI/CVD process after said pressure gradient CVI/CVD processand before said step of subsequently densifying said porous structure.14. The process of claim 1, wherein said step of heat treating saidporous structure follows said pressure gradient CVI/CVD process withoutremoving said porous structure from said CVI/CVD furnace.
 15. A CVI/CVDprocess, comprising the steps of: partially densifying a multitude ofannular fibrous carbon structures within a CVI/CVD furnace by depositinga first carbon matrix within said annular fibrous carbon structure witha pressure gradient CVI/CVD process in which a first portion of eachannular fibrous carbon structure is subjected to a higher pressure thana second portion of each annular fibrous carbon structure and said firstportion has a greater bulk density gain than said second portion; and,subsequently densifying said multitude of annular fibrous carbonstructures by depositing a second carbonaceous matrix within eachannular fibrous carbon structure with at least one additionaldensification process in which said second portion has a greater bulkdensity gain than said first portion.
 16. The process of claim 15,wherein each annular fibrous carbon structure has two generally parallelplanar surfaces, and said first portion includes one of said twogenerally parallel planar surfaces, and said second portion includes theother of said two generally parallel planar surfaces.
 17. The process ofclaim 15, wherein each annular fibrous carbon structure has an insidecircumferential surface and an outside circumferential surface, saidfirst portion includes said inside circumferential surface, and saidsecond portion includes said outside circumferential surface.
 18. Theprocess of claim 15, wherein each annular fibrous carbon structure hasan inside circumferential surface and an outside circumferentialsurface, and said first portion includes said outside circumferentialsurface, and said second portion includes said inside circumferentialsurface.
 19. The process of claim 15, wherein each annular fibrouscarbon structure has an inside circumferential surface and an outsidecircumferential surface, and said first portion includes said insidecircumferential surface, and said second portion includes said outsidecircumferential surface.
 20. The process of claim 15, wherein eachannular fibrous carbon structure has two generally parallel planarsurfaces bounded by an inside circumferential surface and an outsidecircumferential surface spaced from and encircling said insidecircumferential surface, and said first portion includes said insidecircumferential surface and one of said two generally parallel planarsurfaces, and said second portion includes said outside circumferentialsurface and the other of said two generally parallel planar surfaces.21. The process of claim 15, wherein each annular fibrous carbonstructure has two generally parallel planar surfaces bounded by aninside circumferential surface and an outside circumferential surfacespaced from and encircling said inside circumferential surface, and saidfirst portion includes said outside circumferential surface and one ofsaid two generally parallel planar surfaces, and said second portionincludes said inside circumferential surface and the other of said twogenerally parallel planar surfaces.
 22. The process of claim 15, furthercomprising the step of heat treating said annular fibrous carbonstructure at a temperature greater than said pressure gradient CVI/CVDprocess after said pressure gradient CVI/CVD process and before saidstep of subsequently densifying said annular fibrous carbon structure.23. The process of claim 22, wherein said step of heat treating saidporous structure follows said pressure gradient CVI/CVD process withoutremoving said porous structure from said CVI/CVD furnace.
 24. A CVI/CVDprocess, comprising the steps of: heating a porous carbon structure to atemperature of at least 1750° F.; heating a hydrocarbon reactant gas toa temperature of at least 1650° F.; partially densifying said porouscarbon structure by forcing said reactant gas to pass through saidporous carbon structure from a first portion of said fibrous structureto a second portion of said porous carbon structure, said first portionhaving a greater bulk density gain than said second portion; and,subsequently densifying said porous carbon structure by depositing asecond matrix within said porous carbon structure with at least oneadditional densification process in which said second portion has agreater bulk density gain than said first portion.
 25. The process ofclaim 24, wherein said second matrix is a carbon matrix and saidadditional densification process is a conventional CVI/CVD process. 26.The process of claim 24, wherein said additional densification processcomprises the steps of: heating a porous carbon structure to atemperature of at least 1750° F.; heating a hydrocarbon reactant gas toa temperature of at least 1650° F.; and, passing said reactant gasaround said porous structure.
 27. The process of claim 24, furthercomprising the step of heat treating said porous carbon structure at atemperature of at least 3300° F. after said step of partially densifyingsaid porous carbon structure and before said step of subsequentlydensifying said porous carbon structure.
 28. The process of claim 27,wherein said step of heat treating said porous structure follows saidpressure gradient CVI/CVD process without removing said porous carbonstructure from said CVI/CVD furnace.
 29. A CVI/CVD process in a CVI/CVDfurnace, comprising the steps of: introducing a reactant gas into asealed preheater disposed within said CVI/CVD furnace, said sealedpreheater having a preheater inlet and a preheater outlet, said reactantgas being introduced into said preheater inlet and exiting said sealedpreheater through said preheater outlet and infiltrating at least oneporous structure disposed within said CVI/CVD furnace; heating said atleast one porous structure; heating said sealed preheater to a preheatertemperature greater than said reactant gas temperature; sensing a gastemperature of said reactant gas proximate said outlet; adjusting saidpreheater temperature to achieve a desired gas temperature; and,exhausting said reactant gas from said CVI/CVD furnace.
 30. The processof claim 29, wherein said CVI/CVD furnace comprises a susceptor wall,and further comprising the step of heating said susceptor wall, and saidstep of heating said sealed preheater comprises the step of radiatingheat energy from said susceptor wall to said sealed preheater.
 31. Theprocess of claim 29, wherein said sealed preheater is disposed in closeproximity to said susceptor wall.
 32. The process of claim 29, whereinsaid CVI/CVD furnace comprises a susceptor wall having at least firstand second susceptor wall portions and at least first and secondinduction coils, said first induction coil being inductively coupled tosaid first susceptor wall in a manner that transforms electrical energyfrom said first induction coil to heat energy in said first susceptorwall, and said second induction coil being inductively coupled to saidsecond susceptor wall portion in a manner that transforms electricalenergy from said second induction coil to heat energy in said secondsusceptor wall portion, said sealed preheater being disposed proximatesaid first susceptor wall portion and being heated to said preheatertemperature at least in part by radiant heat energy from said firstsusceptor wall portion; and wherein said step of adjusting said heatexchanger temperature comprises the step of adjusting electrical powerto said first induction coil.
 33. The process of claim 29, wherein saidCVI/CVD furnace comprises a cylindrical susceptor wall having at leastfirst and second cylindrical susceptor wall portions and at least firstand second cylindrical induction coils, said first cylindrical inductioncoil being concentrically disposed around and inductively coupled tosaid first cylindrical susceptor wall in a manner that transformselectrical energy from said first cylindrical induction coil to heatenergy in said first cylindrical susceptor wall, and said secondcylindrical induction coil being concentrically disposed around andinductively coupled to said second cylindrical susceptor wall portion ina manner that transforms electrical energy from said second cylindricalinduction coil to heat energy in said second cylindrical susceptor wallportion, said sealed preheater defines a generally cylindrical preheaterperimeter concentrically disposed within and in close proximity to saidfirst cylindrical susceptor wall portion and being heated to saidpreheater temperature at least in part by radiant heat energy from saidfirst cylindrical susceptor wall portion; and wherein said step ofadjusting said preheater temperature comprises the step of adjustingelectrical power to said first induction coil.
 34. The process of claim29, wherein said CVI/CVD furnace comprises a generally cylindricalsusceptor wall, and said sealed preheater comprises a generally arcuatepreheater perimeter in close proximity to said cylindrical susceptorwall.
 35. The process of claim 29, wherein said sealed preheater isresistance heated by electrical energy.
 36. The process of claim 29,wherein said preheater outlet comprises an array of perforations. 37.The process of claim 29, wherein said at least one porous structurecomprises a first portion and a second portion; and, further comprisingthe step of forcing said reactant gas to pass through said at least oneporous structure from said first portion to said second portion.
 38. Theprocess of claim 37, wherein said reactant gas deposits a carbon matrixhaving a substantially rough laminar microstructure within said at leastone porous structure.
 39. The process of claim 29, wherein said at leastone porous structure is a carbon porous structure, and said reactant gasdeposits a carbon matrix within said at least one porous structure. 40.The process of claim 29, wherein said at least one porous structurecomprises a plurality of annular porous structures disposed in a stackthat defines an annular porous wall; and, further comprising the step offorcing dispersion of said reactant gas through said annular porous wallby introducing said reactant gas to said CVI/CVD furnace and exhaustingsaid reactant gas from said CVI/CVD furnace on opposite sides of saidannular porous wall.
 41. The process of claim 40, wherein each annularporous structure has a surface area, said stack of annular porousstructures having at least one ring concentrically disposed between eachpair of adjacent porous structures with a majority of said surface areaof each annular porous structure being exposed to said reactant gas. 42.The process of 40, wherein said stack defines an enclosed cavity boundedby said annular porous wall; and, further comprising the step ofintroducing said reactant gas from said preheater outlet into saidenclosed cavity, said enclosed cavity being sealed to said preheateroutlet.
 43. A CVI/CVD process in a CVI/CVD furnace, comprising the stepsof: forming an annular porous wall that defines an enclosed cavity, saidporous annular wall including a stack of annular fibrous carbonstructures; sealing said annular porous wall to a sealed preheater, saidsealed preheater having a preheater inlet and a preheater outlet, saidenclosed cavity being in fluid communication with said gas outlet;introducing a carbon bearing reactant gas into said preheater inlet,directing said gas through said sealed preheater, to said preheateroutlet, and into said enclosed cavity; heating said annular porous wall;heating said preheater to a preheater temperature greater than a gastemperature of said reactant gas proximate said preheater inlet; sensinga gas temperature of said reactant gas proximate said preheater outlet;adjusting said preheater temperature to achieve a desired gastemperature; and, withdrawing said reactant gas from said CVI/CVDfurnace on a side of said annular porous wall opposite said enclosedcavity thereby forcing dispersion of said reactant gas introduced intosaid enclosed cavity through said annular porous wall.
 44. The processof claim 43, wherein said CVI/CVD furnace comprises a susceptor wallhaving at least first and second susceptor wall portions and at leastfirst and second induction coils, said first induction coil beinginductively coupled to said first susceptor wall in a manner thattransforms electrical energy from said first induction coil to heatenergy in said first susceptor wall, and said second induction coilbeing inductively coupled to said second susceptor wall portion in amanner that transforms electrical energy from said second induction coilto heat energy in said second susceptor wall portion, said preheaterbeing disposed proximate said first susceptor wall portion and beingheated to said preheater temperature at least in part by radiant heatenergy from said first susceptor wall portion; and wherein said step ofadjusting said preheater temperature comprises the step of adjustingelectrical power to said first induction coil.
 45. The process of claim43, wherein said reactant gas deposits a carbon matrix having asubstantially rough laminar microstructure within said annular porouswall.
 46. The process of claim 43, wherein each annular fibrous carbonstructure has a surface area, said stack having at least one ringconcentrically disposed between each pair of adjacent annular fibrouscarbon structures with a majority of said surface area of each annularfibrous carbon structure being exposed to said reactant gas.
 47. ACVI/CVD densification process, comprising the steps of: densifying afirst porous wall within a CVI/CVD furnace by a pressure gradientCVI/CVD process wherein a first flow of reactant gas is forced todisperse through said first porous wall; densifying a second porous wallby a pressure gradient CVI/CVD process wherein a second flow of reactantgas is forced to disperse through said second porous wall; and,independently controlling said first flow of said reactant gas and saidsecond flow of said reactant gas.
 48. The process of claim 47, furthercomprising the steps of: densifying at least a third porous wall by apressure gradient CVI/CVD process wherein at least a third flow ofreactant gas is forced to disperse through at least said third porouswall; and independently controlling at least said third flow of reactantgas.
 49. The process of claim 47, further comprising the steps of:sensing a first porous wall temperature; and, controlling said firstporous wall temperature by increasing or decreasing said first flow ofreactant gas.
 50. The process of claim 49, further comprising the stepsof: sensing a second porous wall temperature; and, controlling saidsecond porous wall temperature by increasing or decreasing said secondflow of reactant gas.
 51. The process of claim 47, wherein: said step ofdensifying said first porous wall includes the step of subjecting oneside of said first porous wall to said first flow of reactant gas at afirst pressure and an opposing side of said first porous wall to avacuum pressure, said first pressure being greater than said vacuumpressure; and, said step of densifying said second porous wall includesthe step of subjecting one side of said second porous wall to saidsecond flow of reactant gas at a second pressure and an opposing side ofsaid second porous wall to a vacuum pressure, said second pressure beinggreater than said vacuum pressure.
 52. The process of claim 51, furthercomprising the steps of: sensing said first pressure; and, controllingsaid first pressure by increasing or decreasing said first flow ofreactant gas.
 53. The process of claim 52, further comprising the stepsof: sensing said second pressure; and, controlling said second pressureby increasing or decreasing said second flow of reactant gas.